Flexible printed wiring has been utilized for many years by numerous industries. At first, flexible printed wiring was utilized in aerospace applications, and more recently, the flexible printed wiring has been implemented in many consumer products. Flexible printed wiring applications range from digital watches to commercial aircraft components, and from domestic appliances and automobiles to deep space hardware.
Flexible printed wiring provides inherent advantageous characteristics including: low weight and volume, increased reliability, flexibility and simplified assembly. Flexible printed wiring encompasses a random arrangement of printed conductors using a flexible insulating base or substrate material. A plurality of cover layers may be provided on the flexible substrate material.
The random arrangement of conductors distinguishes flexible printed wiring from collated, flat flexible cable. The capacity of flexible printed wiring circuits to bend requires that the conductors, adhesive, and cover layer materials utilized in the circuit be flexible similar to the base material.
Flexible printed wiring may comprise various combinations of base, conductor, and cover layers. For example, single-sided flexible printed wiring has conductors on one side of a base layer. Double-sided flexible printed wiring includes conductors on both sides of the base layer. Single access flexible printed wiring includes a given conductor layer accessible from an external connection on one side. Double access flexible printed wiring includes a conductor layer accessible via an external connection from either the conductor side or the base side thereof.
Multi-layer flexible printed wiring includes more than two conductor layers laminated together with insulating base layers between the conductive layers. Rigid-flex flexible printed wiring includes two or more rigid sections having one or more flexible sections provided therebetween. Rigidized flexible printed wiring includes a plurality of rigid sheet material pieces selectively bonded to the flexible printed wiring.
The utilization of a single flexible printed wiring array reduces the number of terminals and soldered joints required for combining component mounting areas with conventional interconnecting cables. Further, plated-through holes between conductored layers in flexible printed wiring are more reliable than the soldered joints and edge connectors which they can replace in conventional connection devices.
Beneficial characteristics of flexible printed wiring include inherent improved flexibility and lower mass per length which reduce strain on soldered joints. These characteristics of flexible printed wiring provide circuits of enhanced reliability compared to round wire when subjected to shocks and vibrations. Flexible printed wiring has increased resistance to damage and flexure when compared with conventional round wire because the conductor material can be positioned closer to the neutral surface and because the bond between the conductors and insulation is uniformly distributed over a larger area.
Flexible printed wiring typically requires special pallets or fixtures, commonly referred to as processing carriers, which are utilized to position and hold the flexible printed wiring terminals during component placement, mass soldering, and testing. These processing carriers essentially support the flexible sheets of material upon which the integrated circuitry is patterned, or traces are formed. The thin flexible sheets are subjected to various processing steps including large heating steps, air drying steps, and printing steps. The thin, flexible nature of the polyester films, makes it extremely difficult, if not impossible, for the flexible sheets to be processed without being received upon a rigid temporary substrate during manufacture.
Various methods have been utilized to temporarily affix the flexible sheets to the processing carriers during the formation of the printed wiring thereon. One prior art method of attachment employs vacuum suction-like cups which are used to temporarily grasp portions of the backside of the flexible circuit substrate.
Alternately, bent pins have been utilized to hold the flexible substrate to the processing carrier. For example, upward pins may be provided at the edges of the flexible sheets. The upward pins may be bent over to grasp the outer surface of the flexible substrate to secure the flexible substrate for processing.
A plurality of holes may be provided within the flexible substrate for the sole purpose of facilitating the attachment of the flexible substrate to the processing carrier during the formation of the flexible printed wiring. Pins extend upwardly from the processing carrier through these holes. A plurality of securing devices, referred to as buttons, are positioned and pushed down upon the pins and onto the outer surface of the flexible substrate. The buttons are removed at the end of the processing of the flexible substrate.
Providing attachment through the use of such tooling pins makes certain processing steps impossible. For example, stencil printing of the flexible circuit is either difficult or impossible without damaging the stencil. Additionally, tooling is difficult to maintain when the tooling pins are utilized to secure the flexible circuit substrate.
In all of these prior art techniques, only some portion of the backside of the flexible sheet is actually retained or held fast to the processing carrier. Even where a screen is utilized as a support for a vacuum on the backside of the flexible substrate, an adhesive force is not provided at the portions where the screen physically touches the flexible substrate. In addition, the thin flexible circuits may "dimple" and stretch when vacuum is transmitted through a handling panel operating to hold the flexible circuit during processing.
Further, the utilization of a vacuum to hold the flexible circuit substrate may only be utilized at a single piece of processing equipment and may not be utilized to hold the substrate when the holding panel is transferred between various pieces of processing equipment.
One conventional technique for affixing the flexible circuit substrate to the processing carrier includes the external taping of corners of the upper surface of the substrate to the holding panel. However, such a method prevents processing of the portions of the substrate which are beneath the external tape. Further, such a method fails to prevent airflows created by the processing equipment (e.g., curing oven) from lifting an unrestrained area of the flexible circuit. Still further, taping of the corners of the substrate during certain processing steps, such as screen printing, is undesirable inasmuch as the film has a tendency to stick to the screen or stencil and lift upwards way from the rigid processing carrier.
In addition, further processing steps may not be immediately performed when the prior art methods of attachment are utilized. In particular, the outer surface of the flexible substrate sheets cannot be encapsulated on the processing carrier if prior art pins and/or external taping are utilized for affixing the flexible sheets. Specifically, the external tape attached to the top of the flexible substrate, or the pins protruding through the upper surface, or the buttons coupled with the pins, would be completely encapsulated precluding practical removal from the processing carriers.
Therefore, there exists a need for providing improved methods for securing flexible circuit substrates against the processing carrier during the formation of flexible printed wiring circuits.